Moyamoya disease is an obliterative vasculopathy of the large arteries at the base of the brain. In the United States, it most commonly affects women in their third and fourth decades of life, causing ischemic stroke. The natural history of this disorder is not defined. It is very likely that hemodynamic factors play an important role in the risk of future stroke. Some patients are treated surgically with a variety of procedures intended to improve collateral blood flow to the brain. The effect of these procedures on cerebral blood flow and on subsequent stroke risk reduction is unknown. This blinded, prospective study will investigate the role of cerebral hemodynamic factors in the pathogenesis of stroke in patients with moyamoya disease. We will test the hypothesis that increased oxygen extraction in the cerebral hemisphere distal to the occlusion is a predictor of the subsequent risk of ipsilateral ischemic stroke in medically treated patients (Specific Aim 1). The secondary objectives of this project are (1) to determine other predictive factors for stroke in this population;(2) to obtain preliminary data on the effects of different medical treatment regimens in this population;(3) to determine the temporal changes in hemodynamic impairment in medically treated patients;(4) to determine the effects of surgery on hemodynamic impairment in the subset of patients that undergo surgical revascularization;and (5) to obtain estimates of surgical complication rates for patients with and without hemodynamic impairment. These objectives will be accomplished by the use of positron emission tomography (PET) to measure regional cerebral oxygen extraction fraction (OEF) in patients with moyamoya disease. Information on clinical, epidemiologic and angiographic risk factors will be obtained on enrollment. Patients are subsequently followed every six months by telephone to determine the subsequent occurrence of stroke and the interim treatment for stroke prevention. Results of the PET study are kept confidential from the patient, treating physician and investigator monitoring the occurrence of stroke. We will determine if a regional increase in OEF is associated with an increased risk of ipsilateral ischemic stroke. We will also do a multivariate analysis to determine the independent predictors of stroke in this sample. We will perform a post-hoc longitudinal analysis of the effect of different medical treatment regimens. Patients will return annually for follow up PET studies.